Boolean union of multiple objects
For every boolean, i use the add-on BoolTool, go to user preferences > Add-ons > search Bool Tool :

Then i create 5 cubes with crossing areas : 


Select all your cubes with Ctrl+A, then use the Bool Tool Union operator : 
- Go to object > Bool Tool Operators > Union
Or
- Ctrl+Shift+C > Union

You should get this result :